Environmental Protection Agency (EPA) regulations now require leak detection equipment for underground storage tanks containing hydrocarbon-bearing liquid such as fuel oil, heating oil and gasoline. The EPA estimates that there are about two million tanks at 750,000 locations in the United States, and that up to one-fifth of those tanks are leaking.
A main problem with any leakage detection system is to compensate for changes in the volume of the stored liquid due to temperature changes. Because hydrocarbon-bearing fluids have a relatively high coefficient of expansion, a decrease in temperature can result in a significant decrease in the volume of liquid in the tank, without any leakage. Of course, without compensating for this change, it is not possible to distinguish a leak from a level change caused by a temperature change.
Various equipment has been proposed for measuring the fluid in a tank and for detecting leakage. For example, U.S. Pat. Nos. 4,244,218 and 4,387,778 to Wohrl are directed to fluid measuring devices including a plunger connected to a load cell and extending into the liquid in the tank. However, the equipment discussed in these patents requires extensive calibration. U.S. Pat. No. 3,677,356 to Goodwin also is directed to the weighing of liquids and includes a body submerged in the liquid to be measured. This body is supported by one end of a balance beam.
U.S. Pat. No. 4,300,388 to Hansel et al. shows leakage measurement equipment including a sensor extending into the liquid and supported by a balance. The sensor includes a cup portion for compensating for evaporation of the hydrocarbon in the tank. The sensor is filled with liquid and careful calibration is required.
U.S. Pat. Nos. 4,453,400; 4,604,893 and 4,630,467 to Senese et al. are all directed to a leak detector and method of use in which light from a bulb is directed at a photoresistor. While the bulb and photoresistor are held fixed, the photoresistor is submerged in a cup of India ink solution held at the top of a float. Minute vertical movement of the float is sensed by the photoresistor because the level of the solution covering the photoresistor varies in accordance with the float position.